A War Declared Over Two Jinchūriki
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Five Kage Summit · Naruto & Killer B Withheld · Obito Declares War
Obito does not declare the Fourth Shinobi World War because the Five Great Nations invade Akatsuki territory. He declares it after the Kage refuse to surrender Naruto and Killer B, the two remaining jinchūriki he still needs for the Eye of the Moon Plan. The war begins as a demand for two people and immediately forces villages with generations of hostility to decide whether protecting them is worth fighting together.
What changed
The Five Kage Summit was supposed to coordinate a response to Akatsuki. It ends by creating something the shinobi world had never sustained before: a military alliance of the major villages against a common enemy.
Five Villages Become One Army
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Former Enemies · Shared Headbands · A Single Command Structure
The alliance is more radical than simply putting five armies on the same map. Shinobi who were raised under different village doctrines, fought in earlier wars, and in some cases personally hated one another are reorganized under a shared command structure. The old village symbols do not disappear, but for the duration of the war they stop being the most important identity on the battlefield.
Why it matters
Akatsuki spent years exploiting the distrust between villages. The war forces those same villages to pool intelligence, manpower and leadership quickly enough that old political divisions cannot be allowed to decide battlefield decisions.